This invention relates generally to the preparation of higher olefins using a chain growth reaction of a lower olefin and especially ethylene with a low molecular weight alkyl aluminum and more specifically to such processes using an improved butene displacement reaction to displace the higher olefins from the aluminum, which permits efficient operation of the displacement reaction at pressures where the reaction mixture is in the two-phase, vapor-liquid or the vapor-phase regions.
Alpha-olefins are made in commercial quantities by a process initially developed in the fifties by Karl Ziegler and his co-workers. The so-called Ziegler process involves the reaction of triethyl aluminum ("TEA") and ethylene at temperatures in the range of 200.degree.-500.degree. F. and pressures in the ran 2000-5000 psig to yield a mixture of tri-C.sub.2-20 + alkyl aluminum having a Poisson alkyl distribution and C.sub.2-20 olefins. The ethylene is flashed from the reaction mixture for recycle and the light olefins through decene-1 can be distilled from the mixed aluminum alkyls since they have a normal boiling point below the lightest aluminum alkyl (viz. TEA).
Johnson, U.S. Pat. No. 2,863,896, describes the preparation of pure aluminum alkyls using a chain growth reaction of a C.sub.2-4 olefin (e.g. ethylene) with a low molecular weight trialkyl aluminum (e.g. TEA), dialkyl aluminum hydride or alkyl aluminum dihydride. The chain growth product contained about 2-5 percent C.sub.4-20 olefin which could not be separated from the aluminum alkyls. The mixture was then subjected to a displacement reaction with a C.sub.4-6 .alpha.-olefin, e.g. 1-butene, to displace mainly C.sub.6-20 .alpha.-olefin forming tributyl aluminum. The C.sub.6-20 .alpha.-olefins were fractionated into individual .alpha.-olefins. These individual .alpha.-olefin cuts were then reacted in a second displacement reaction with the tributyl aluminum formed in the butene displacement reaction to form pure trialkyl aluminum.
Catterall et al., U.S. Pat. No. 2,889,385, describes an ethylene chain growth reaction carried out on tributyl aluminum followed by displacement with 1-butene to regenerate tributyl aluminum and evolve C.sub.4-20 .alpha.-olefins. This avoids the problem encountered in attempting to separate TEA from C.sub.12-14 .alpha.-olefins which have normal boiling points close to the same temperature.
Other patents disclosing variations of the aluminum alkyl chain growth .alpha.-olefin synthesis are U.S. Pat. Nos. 2,906,794; 2,971,969; 3,180,881; 3,210,435; 3,227,773; 3,278,633; 3,352,940; 3,663,647; 3,789,081; 3,487,097, 4,314,090; 3,359,292; 3,384,651; 3,415,861; 3,458,594 and 3,358,050.
All aluminum alkyl chain growth products initially yield a mixture of higher trialkyl aluminum compounds exhibiting a Poisson distribution. When ethylene and TEA are used, the mixture is mainly tri-C.sub.2-20 alkyl aluminum compounds although a small amount of C.sub.20+ alkyls are usually present. Ethylene displacement of this mixture yields a mixture of C.sub.2-20 olefins. The more valuable components are the C.sub.6-14 .alpha.-olefins. Ethylene can be recycled to chain growth and olefins above C.sub.14 can be separated for use as diluent or can be purged. Butene is produced in fairly large amounts and cannot economically be discarded. The commercial market for butene is substantially saturated. Copending application Ser. No. 373,247 provides a process that utilizes the butene that it produces in a closed butene displacement loop that gives high yield of the more desirable C.sub.6-12 .alpha.-olefins. The displacement reaction is carried out at pressures of about 1000 to 2000 psig. Operation at lower pressures where the reaction mixture passes from a single liquid phase to a two-phase vapor-liquid or a vapor-phase system has been found to result in breaking down of the aluminum alkyl with resulting formation of deposits on the reactor walls which may even plug a small diameter tubular reactor.
We have discovered that by providing enhanced radial mixing of the reactants, the displacement reaction can be smoothly carried out at a range of pressures from 2000 psig down to at least about 65 psig without deposits. This permits the use of relatively low pressure butene displacement.